An electromagnetic interference between devices and a noise propagating GND (ground) or a power supply may cause a malfunction in a circuit board, an SOC (system on chip), and a pseudo SOC, on each of which a digital-analog-RF mixed circuit is mounted. In order to avoid the problem, it is necessary to increase a space between the devices. Therefore, a chip area or a mounting area is hardly reduced.
A filter preventing the noise in a frequency domain, which propagates to the GND or the power supply and is harmful to a circuit operation, is used as means for reducing the space. In this case, the filter reducing a noise level of the restricted frequency domain that is of a cutoff band is used. For example, the filter is formed using electronic components such as an inductor chip and a capacitor. Accordingly, it is inevitable to increase the number of electronic components mounted on the circuit board.
Therefore, it has been proposed that an EBG (Electromagnetic Band Gap) structure is provided in an dielectric board, because an effect to reduce the circuit board is enhanced compared with the filter with electronic components. There is a demand to further improve an electromagnetic cutoff characteristic of the EBG structure.
For the EBG structure, in the case that the cutoff band is set to a low-frequency domain (6 GHz or less), unfortunately the EBG structure is enlarged, and the circuit board hardly downsized. Additionally, there is a problem in that a wide stop band is hardly ensured.
On the other hand, an effort to improve the EEC structure is implemented in order to enhance a characteristic of the EBG structure or to reduce a size of the EBG structure.